Oncolytic viruses are generally selected or engineered to grow inside cancer cells preferentially, as compared with normal cells (Kim et al., 2001). A wide variety of oncolytic viruses have been used in preclinical and clinical cancer therapies (see Parato et al., 2005; Bell et al, 2003; Everts and van der Poel, 2005; Ries and Brandts, 2004). These viruses can cause tumor cell death through direct replication-dependent and/or viral gene expression-dependent oncolytic effects (Kim et al., 2001). In addition, viruses may be able to enhance the induction of cell-mediated antitumoral immunity within the host (Todo et al., 2001; Sinkovics et al., 2000). These viruses also can be engineered to expressed therapeutic transgenes within the tumor to enhance antitumoral efficacy (Hermiston, 2000).
Oncolytic viruses that have been selected or engineered to productively infect tumour cells include adenovirus (Xia et al., 2004; Wakimoto et al., 2004); reovirus; herpes simplex virus 1 (Shah, et al., 2003); Newcastle disease virus (NDV; Pecora, et al., 2002); vaccinia virus (VV; Mastrangelo et al., 1999; US 2006/0099224); coxsackievirus; measles virus; vesicular stomatitis virus (VSV; Stojdl, et al., 2000; Stojdl, et al., 2003); Seneca Valley Virus (Reddy, et al. 2007), influenza virus; myxoma virus (Myers, R. et al., 2005).
A variety of mechanisms have been suggested for mediating tumor selectivity in oncolytic therapies. Methods of targeting oncolytic viruses to cancer cells may, for example, be based on differential expression of receptors. An unmodified cocksackie virus (CAV21) is thought to selectively target cancer cells because the viral receptors ICAM-1 and DAF are overexpressed on malignant melanoma cells. WO 2005/087931 discloses selected Picornavirus adapted for lytically infecting a cell in the absence of intercellular adhesion molecule-1 (ICAM-1). Engineering to alter specificity of receptors is also possible to enhance tumor-targeting of onocolytic viruses including adenovirus (Sebestyen 2007, Carette 2007) and measles (Allen 2006, Hasegawa 2006), for example.
Alterations in cancer cell signalling pathways lead to the upregulation of genes required for cell proliferation, such as thymidine kinase (TK) and ribonucleotide reductatase (RNR) which are required for the production of dNTPs used in DNA synthesis. The replication of viruses can be restricted to cancer cells with high TK or RNR activity by deleting the viral versions of TK or RNR, or other genes upregulated in cancer cells. U.S. Pat. No. 7,208,313 discloses a VV with TK and VGF deletions and WO 2005/049845, WO 2001/053506, US 2004/120928, WO 2003/082200, EP 1252323 and US 2004/9604 disclose herpes viruses such as HSV, which may have improved oncolytic and/or gene delivery capabilities.
The Ras/PRK pathway is commonly mutated in cancer cells. This pathway affects the interferon (IFN) response pathway, which is thought to be deficient in cancer cells with activated Ras. Some oncolytic viruses are tumor-selective based on the difference in the innate immune response pathway in normal vs. cancer cells. For example, unmodified reovirus is naturally selective to cancer cells based on the lack of innate anti-viral response in tumor cells with activations in the Ras/PRK pathway. WO 2005/002607 discloses the use of oncolytic viruses to treat neoplasms having activated PP2A-like or Ras activities, including combinations of more than one type and/or strain of oncolytic viruses, such as reovirus. It has been suggested that sensitivity to a host interferon response is a desirable characteristic for oncolytic viruses. For example, WO 2004/014314 suggests that poxviruses, including vaccinia virus, may be adapted for use to treat cancers by introducing mutations in genes encoding interferon binding proteins.
Susceptibility to an anti-viral response may, however, confer safety benefits on an oncolytic virus. For example, intranasal infection of mice with VSV has been shown to lead to lethal infection of the CNS. This serious adverse effect may be addressed by combined treatment with interferon and VSV (Stojdl et al., 2003). In an effort to find additional mechanisms to address the risks posed by therapeutic uses of systemic VSV infection, VSV strains have been developed that have a heightened sensitivity to a host interferon-mediated anti-viral response. In particular, amino acid substitutions in the matrix (M) protein gene of VSV, such as VSVdeltaM51 (Lun et al., Journal of the National Cancer Institute 2006 98(21):1546-1557), may be stronger inducers of the host interferon response, and may be vulnerable to the intact interferon response of normal cells but not to the attenuated interferon response of cancer cells which are deficient in IFN response pathways. EP 1218019, US 2004/208849, US 2004/115170, WO 2001/019380, WO 2002/050304, WO 2002/043647 and US 2004/170607 disclose oncolytic viruses, such as Rhabdovirus, picornavirus, and VSV, in which the virus may exhibit differential susceptibility to a host interferon response, with selectivity for tumor cells having low PKR activity.
VV expresses a repertoire of mechanisms for evading host anti-viral responses. At least partly as a consequence of these attributes, VV infection of an immunocompromised host can lead to serious and lethal complications, such as systemic vaccinia infection with encephalitis (Lane, et al., 1969; Arita, et al., 1985). To address the risks entailed in systemic use of wild type VV, strains have been developed that have deficits in various aspects of the VV infection pathway. For example, VV may be engineered to lack thymidine kinase (TK) activity. A TK-strain of VV requires thymidine triphosphate for DNA synthesis, which leads to preferential replication in dividing, and hence cancerous, cells. In an alternative approach, VV strains may be engineered to lack vaccinia virus growth factor (VGF). This secreted protein is produced early in the VV infection process, acting as a mitogen to prime surrounding cells for VV infection (Buller et al., 1988, Virology 164: 182-192). Combined strains of VV having both TK and VGF deficits benefit from the enhanced safety and specificity that results from the absence of mitogenic VGF activity in conjunction with the metabolic host cell specificity conferred by the requirement for thymidine triphophate. Similarly, vaccinia virus engineered to lack genes that normally thwart the host IFN-response (E3L, K3L, B18R, etc.) are also attenuated in normal cells but can grow in tumor cells lacking IFN-response. WO 2005/007824 discloses oncolytic vaccinia viruses and their use for selective destruction of cancer cells, which may exhibit a reduced ability to inhibit the antiviral dsRNA dependent protein kinase (PKR) and increased sensitivity to interferon. WO 2003/008586 similarly discloses methods for engineering oncolytic viruses, which involve alteration or deletion of a viral anti-PKR activity.
A number of orthopoxviruses express a type I interferon (IFN)-binding protein, which is encoded by the B18R open reading frame in the WR strain of vaccinia virus. The B18R protein has significant regions of homology with the subunits of the mouse, human, and bovine type I IFN receptors, reportedly binds human IFN2 with high affinity, and is reported to inhibit transmembrane signaling. The B18R protein reportedly exists as a soluble extracellular as well as a cell surface protein, and thus may block both autocrine and paracrine functions of IFN (Colamonici et al., 1995, JBC vol 270, pp 15974-16978). U.S. Pat. No. 7,285,526 describes therapeutic uses of B18R.
Further references that have considered oncolytic viruses within the scope of tumour biology include the following: Bell, 2007; Parato et al., 2005; Crompton et al., 2007; Russell and Peng, 2007; Park et al., 2008; Liu et al., 2008; Kim, 2001; Kumar et al., 2008; McCart et al., 2001; Parr et al., 1997; Thorne et al., 2007; Symons et al., 1995; Alcami and Smith, 1995; Colamonici et al., 1995; Alcami et al., 2000; Stojdl et al., 2000; Stojdl et al., 2003; and Lichty et al., 2004.
Abbreviations used herein include the following: eGFP, enhanced green fluorescent protein; MOI, multiplicity of infection; PFU, plaque-forming unit; FAST, Fusion Associated Small Transmembrane; SEM, standard error of the mean; DsRed, Discosoma sp. red fluorescent protein; and IU, international units.